1. Field of the Invention
The present invention relates to a semiconductor test's apparatus (image sensor tester) for carrying out a performance test of a CCD or a CMOS constituting an image sensor and to a performance board provided in this semiconductor test apparatus. More particularly, it relates to a semiconductor test apparatus and a performance board suitable to correct the gain, etc. of a buffer provided to drive a cable connected between the performance board and a capture board.
2. Description of the Related Art
An image sensor is a device which measures the width and length of an object by use of an image sensor element having aligned photoelectric elements for converting a quantity of light into an electric signal.
This image sensor includes a CCD image sensor and a CMOS image sensor.
The CCD image sensor achieves a transfer by use of a circuit element called a charge coupled device (CCD) in order to read an electric charge generated from light by a light receiving element, when converting an image into an electric signal.
On the other hand, the CMOS image sensor is a solid-state image sensing device using a complementary metal-oxide semiconductor (CMOS), and is characterized by having an amplifier for each unit cell to enable the suppression of the generation of electric noise due to the reading of an optically corrected electric signal converted into light.
Meanwhile, in manufacturers of the image sensors, a performance test is carried out using an image sensor tester in order to check whether a manufactured image sensor has desired performance.
Various improvements have heretofore been proposed for this image sensor tester, for example, by Japanese Patent Publication Laid-open No. 5-56460 and Japanese Patent Publication Laid-open No. 2002-99751.
Here, the configuration of the image sensor tester will be described with reference to FIG. 3. FIG. 3 is a block diagram showing the configuration of the image sensor tester.
When a CCD element which is one of the image sensors is tested, an analog signal of an analog system 110 of a CCD driving unit and a timing signal from a timing system 120 of the CCD driving unit are supplied to a CCD element under test mounted on a performance board 130. An output (measurement signal) of the CCD element under test is converted from an analog signal to a digital signal by an A/D converter 141 of an analog capture board 140, and once loaded into an image data memory 150 of an image processing unit. The data loaded into the image data memory 150 is processed by an image processing unit 160, and a result of the processing is stored in an output memory 170. A synchronization signal of a television signal is generated in a synchronization signal generating unit 180, and this synchronization signal is supplied to a television monitor 190, and the data is read from the output memory 170 in synchronization with the synchronization signal and then supplied to the television monitor 190 on which the data is shown.
Such an operation makes it possible to carry out the performance test of the analog output of the CCD element.
When such a performance test of the CCD element is carried out, it is necessary to perform calibration in the image sensor tester to correct the gain of the amplifier in the analog capture board.
A configuration for performing this calibration will be described with reference to FIG. 4.
As shown in FIG. 4, in the performance board 130 of an image sensor tester 100, buffers 131-1 to 131-n are connected to analog output terminals of CCD elements (devices under measurement).
The buffers 131-1 to 131-n drive cables 200-1 to 200-n connected between the performance board 130 (measurement signal output terminals 132-1 to 132-n) and analog capture boards 140-1 to 140-n (measurement signal input terminals 142-1 to 142-n). The reason is that analog output lines of the devices under measurement have such high output impedances that the cables 200-1 to 200-n can not be driven, which needs to be compensated for.
On the other hand, the analog capture boards 140 (CH1 to CHn) comprise the measurement signal input terminals 142-1 to 142-n to which the measurement signals from the devices under measurement are input, amplifiers (amplifier circuits) 143-1 to 143-n for amplifying the input measurement signals, and A/D converters (ADCs) 141-1 to 141-n for the analog-to-digital conversion of the measurement signals. Thus, the measurement signals output from the devices under measurement can be amplified, analog-to-digital-converted, and then output.
Furthermore, the analog capture board 140 comprises reference signal input terminals 144-1 to 144-n to which a reference signal from a reference signal generator 300 is input, switches (relays) 145-1 to 145-n for switching between the input of the reference signal from the reference signal generator 300 and the input of the measurement signals from the devices under measurement, and D/A converters (DACs) 146-1 to 146-n for correcting the gains of the amplifiers 143-1 to 143-n.
In the image sensor tester having such a configuration, the reference signal generator (reference voltage source) 300 whose output voltage accuracy is high is prepared, and the output of this reference signal generator 300 is applied to the analog capture boards 140-1 to 140-n, thereby correcting the gains of the amplifiers 143-1 to 143-n so that the A/D conversion results in a desired value.
Although, in conventional image sensor testers, the gain of an amplifier provided in an analog capture board can be adjusted, there has been a problem that the gain of a buffer provided in a performance board can not be corrected.
Furthermore, there has been a problem that a voltage offset or timing offset of the buffer can not be corrected either.